The present invention relates to the field of automatic transmissions, and more particularly relates to an automatic transmission which is suitable for use with power sources which have a zero idling speed. In particular, the present invention relates to an automatic transmission suitable for use with vehicles powered with rotary electric motors.
In general, the torque and revolution speed characteristics of an electric motor are better matched with the load characteristics of a vehicle than are those of an internal combustion engine such as a gasoline or a diesel engine. In the case of a railway engine, normally, no changing of gear ratios is necessary for the effective coupling of an electric motor to the wheels, but in the case of an electric automobile the driving and running conditions change over such a relatively wider range, as compared with a railway engine, that changes are required in delivered revolution speed and torque greater than those of which the power source is capable, and therefore a gear changing mechanism or gearbox is required in the transmission of such an electric automobile. For this reason, even in conventional electric cars, an ordinary pedal operated clutch and manual transmission has been used, rather as in a car using a normal internal combustion engine as a power source. Further, it is of course possible to incorporate in an electric car a normal automatic transmission as fitted to a car provided with a normal internal combustion engine power source. However, since for an electric power source there is no need for the power source to idle at a certain non-zero idling speed when the vehicle is temporarily stopped, it is necessary to provide operating oil pressure to the automatic transmission by an auxiliary electric pump or other power source which is kept in operation while the vehicle is temporarily at a standstill. This is expensive, unreliable, and inefficient.
Further, it is to be noted that, because of the good characteristics of an electric motor with regard to torque and revolution speed, it is not normally necessary to provide more than two forward speed stages for an automatic transmission, as opposed to the case of internal combustion engine, in which normally a minimum of three forward speed stages are desirable.